Auto-ignition of gas turbine lubricating oils in a shock tube using spray injection

In choosing the lubricating oil for a gas turbine system, properties such as viscosity, viscosity index, corrosion prevention, and thermal stability are chosen to optimize turbine longevity and efficiency. Another property that needs to be considered is the lubricant’s reactivity, as the lubricant’s ability to resist combustion during turbine operation is highly desirable. In evaluating a method to define reactivity, the extremely low vapor pressure of these lubricants makes conventional vaporization by heating impractical due to the high temperatures and fuel cracking as well as issues with preferential vaporization. To this end, a new experiment was designed and tested to evaluate the reactivity of lubricating oils using an existing shock-tube facility at Texas A&M University equipped with an automotive fuel injector. This experiment disperses a premeasured amount of lubricant into a region of high-temperature air to study auto-ignition. To ensure proper dispersal, a laser extinction diagnostic was used to measure the lubricant particles behind the reflected shock as they are dispersed and vaporized. An OH* chemiluminescence diagnostic measuring light emitted during combustion at around 306 nm was used to determine ignition delay time. Pressure was also measured at the sidewall and endwall positions for test repeatability and exothermicity of the experiments. The methods were validated by conducting experiments with ethanol and comparing the results to previous heated shock-tube experiments conducted in the same facility. Using this method, various 32-, 36-, and 46-weight lubricants identified as widely used in the gas turbine industry were tested. Experiments were conducted in post-reflected shock conditions around 1370 K (2006 ºF) and 1.2 atm, where ignition delay time, peak OH* emission and time-to-peak values were recorded and compared. Ignition was observed for all but one of the lubricants at these conditions, and mild to strong ignition was observed for the other lubricants with varying ignition delay times.